1. Field of the Invention
The present invention relates to molding methods and a cooling apparatus for pressure resistant bottles of synthetic resin, especially polyethylene terephthalate resin (hereinafter called PET).
2. Background of the Invention
Recently, biaxially stretched big PET bottles, such as 1.5 liter bottles, are widely used. These bottles are big foot type petaloid bottles which are the only bottles that have self-standing bottle bodies. Prior bottles are composed of a bottle body having a round bottom portion and a base cup attached to the round bottom portion. The bottom portion of the big foot type petaloid bottle has a complex structure to enable itself to be self-standing and to resist an inner pressure. The big foot type petaloid bottle does not require a base cup, so that, compared to the prior bottle, it has higher productivity and less scrap problem after use.
The distribution of wall thickness of a 1.5 liter pressure resisting bottle is shown in FIG. 4. The central part 6 (a range of 40 .phi. of the undersurface of the bottom portion 3) is thicker than the thickness of the leg portion 3 and the valley portion 5 as shown by line a. The thickness of the central part 6 is more than 2 mm.
The central part 6 of the bottom 3 is required to be thick. When the central part 6 is thin, it lacks mechanical strength and induces crazing and bottom-breakage. This is caused by the high temperature and pressure generated after a substance is filled in the bottle.
Therefore, the bottom part 6 of the bottle should be thicker compared to other parts of the bottle. However, since the bottom part 6 is thicker, it takes longer for the bottom part 6 to cool to a certain temperature (normally, to the temperature of glass transition point of the synthetic resin). When the blowing time is short and the center part 6 is not cooled enough, the center part 6 of the bottle projects outwardly, as shown in FIG. 8, after being released from the mold.
In the past, to prevent deformation of the center part 6, blowing time is set to more than 4 seconds to cool the center part 6 by a blow mold. The temperature characteristic curve b in FIG. 5 shows a relationship between blow time and temperature of the center part 6, which is measured 7 seconds after a bottle is released from a blow mold. It is clear from this temperature characteristic curve b that to prevent deformation of the center part 6 and to cool it below the glass transition point, blow time of longer than 4 seconds is required.
FIG. 6 shows a variation characteristic of blow time which varies depending on the height H (see FIG. 8) between the bottom edged of the leg portion 4 and the center undersurface of the center part 6. In FIG. 6, the characteristic curve C1 indicates that 1 second of blow time is applied, c2 indicates 2 seconds of blow time, c3 indicates 3 seconds of blow time, c4 indicates 4 seconds of blow time, c5 indicates 5 seconds of blow time and c6 indicates 7 seconds of blow time. From experience, crazing and breakage are prevented for a height H of 4.0 mm or more. According to FIG. 6, more than 4 seconds of blow time are required. However, if more than 4 seconds of blow time are applied, productivity fails to improve.
To increase productivity, a number of ideas is discussed, such as to improve the cooling capacity of the blow mold or to make the wall of a blow mold thinner, thereby affecting the cooling agent to the mold face of the blow mold. However, such ideas do not efficiently cool the center part of a bottle while raising the cost for equipment.
Therefore, the object of the invention is to shorten the blow time and improve productivity. Further, the mechanical strength of the bottom portion of the bottle is improved.